Refractory binder for metal casting molds



States Pate- REFRACTORY BINDER FOR METAL CASTING MOLDS John A. Henric ks, Lakewood, Ohio No Drawing. Application June 2, 1954 Serial No.j434,0;67

4 Claims. c1. w t-ass This invention relates to the investment casting of molds and .is a continuation-impart of my co-pending application Serial No. 160,577, filed Mayo, 1-950, now Patent No. 2,682,092, issued June 29, 1954.

For the purposes of illustration the invention isdisclosed herein principally as applied to the art of .precision casting by the lost wax or investment method, its application to other methods of casting to provide greater precision and better surface qualities of castings and better refractory qualities of molds being readily apparent n While plaster of Paris and magnesium .oxyphosphate binders have proven adequate for wax investment casting in the dental art, the wide industrial useof this technique forstainless steel and other chromium alloys has demanded more refractory materials.

It has been found that a silicic acid vsol formed by the hydrolysis of ethyl orthosilicate makes a very satisfactory binder for the refractory powders used to cast high melting ferrous alloys.

It is a principal object of this invention to form colloidal silicic acid binders for refractory powders by utilizing cheap and plentiful water glassas a source of silicic acid in place of the expensive silicic acid alcohol esters now used in the. art.

It is an object of this invention to use these novel inorganic silicic acid'binders alongv with properly proportioned refractory powders to invest wax patterns for precisioncasting.

It is also an object of this invention toutilize these novel binders to make refractorycoressfor metalcasting.

I have found that a diluted water glass can be converted intoa reasonablystable silicic acidv sol by acidulation with phosphoric acid if an excess of phosphoric acid is used in neutralizing the alkali of the sodium silicatesolution, and that such a sol makes an excellent refractory binder when additional refractory cationsare provided for a delayedreaction'with the free phosphoric acid used as stabilizer for the sol. The refractory cations for reaction with the free phosphoric acid of the binder can be provided either as'a basic oxide in the refractorypowder mixture or as a salt of a volatile acid which will be. driven ofi during the steps of mold drying or wax pattern burnout.

The methods are illustrated in the followingv examples.

Example I .Col loidql silica. binder When a number of. different metals. and different types of patterns are to..be cast, I. preferIto use a universal iniifatentecl Apr. 7, 1959 .vestment forall molds, andthenrto precoator fiuxthose which require'special'treatment. flnlillustration of such auniversal investmentis:

ning 5e oz. of a non-ionic a polyeth lerie glycol 9 did??? 2400 cc. o fwater contai wetting agen stearate and about 19:3 octyl alcohol)) 2 The silica powders are first thoroughly blended, and then the liquids are made up. Since phosphoric acid has a considerable heatof dilution, itshould be mixed and cooled be'fo're investing begins. The cooled diluted phosphoric acid is put in a container with stirring means, and the diluted sodium silicate is poured in with stirring. The aeid liquid' A is partially neutralized by the alkali ,of 'liquid B (silicate) with the resultant formation 'ofjmon o'sodium phosphate andcolloidal silicic acid whichin'turn is peptizedbyihe excess phosphoric acid to form a colloidal binder. This reaction also gives oif heat, and the mixer should be cooled for production mixing; :The investment can be mixed in acernent mixer, a cake type mixer, or foundry sand muller, silica powder in each case being siftedintolthe mixed cool liquids.

It is' important that thefinixed investment material be kept in agitation duringfthe' investing, in order that the coarse particles do not .settle out during the flask filling cycle. The poured investmentdoes. not requirea vacuum treatment because of the wetting agent present in the liquid, but it must be tamped or' vibrated around thewax pattern. The usualtampingtable equipment is suitable, but any 60 cycle electrical or mechanical vibrator is satisfactory .to pack the wet investment around the pattern. "A three inch gummed paper. collaris put around the top of the flask or an overlength paper is usedas a liner, so that the excess silica gel can rise above the flask properto eliminate shrinkagei After the investment has ,set' up, the excess collar abovethe flaskiscut ofito leave a dense, homogeneous vflask 'of investment. The flask should preferably'set over night forsb est results, but it .c'antbe'dewaxed in an hour or two after the collar'is cut off. Ifa faster cycle is required, the formation of the .above binder can bevariedithroug hthe following ranges for a-40 pound mix:

The maximum ,concentration is set by the tendency to form too much silica gel, which causes'mold cracking due to silica gel shrinkage when dried. The minimum concentration is determined by. the loss of strength. which in-the case of (3) aboveresults in.a fragile mold that needs an addition of 1 to 2% of an organicbinder, such asdextrin or gum arabic .to I give it sufiicient working strength, or this weak mold may be dewaxed in a solution of aceramicbinder, then oven driedfor strength. The silica solformed by neutralizing .the free alkali of the water glass is too unstable if the neutralization goes beyond the formation of mono sodium phosphate and the pH goes above pH 3.0. The most stable silica sol is formed when suflicient excess acid is used to have one mol of NaH PO and one mol of H PO or equivalent strong acid so in equilibrium, that the pH is kept between 1.0 and 2.0.

The function of the free acid is to peptize the silica sol, so that a minimum acid such as shown in (4) above will set up in half the time of the preferred amount in Example I, as will the substitution of a weak acid such as acetic acid for the phosphoric acid shown.

Example Il.Clloidal silica binder containing salt of refractory amphoteric metal (1) Dry investment powder:

20 pounds of 325 mesh amorphous silica 30 pounds of 140 mesh ground quartz 30 pounds of 80 mesh banding sand Mixed into uniform powder blend. (2) Silicic acid sol binder:

Solution A- 717 ml. of 85% H;P0 mols H PO, 950 ml. of water The acid is diluted with water and cooled. Solution 8-- 1667 ml. of 38%Na,O-3.2Si0 water glass 3333 ml. of water=3 mols Na O =10 mols 5.0 The water glass is diluted with water. Solution C--3333 ml. of 27% ZnCl, solution=8 mols ZnCl,.

As in the previous example, the diluted water glass is run into the diluted phosphoric acid with energetic stirring to form a silicic acid sol, after which addition, the zinc chloride solution is added to the silicic acid-phosphoric acid mixture, also with stirring.

The investment powders are then added to the aqueous binder liquid, using a concrete or other appropriate mixer.

The mixed binder and refractory powder is used to invest wax patterns in the same manner as Example 1.

After the molds havev set up, they are dewaxed either over a steam table or a low temperature oven, or by the novel aqueous dewaxing methods of my co-pending application.

The dewaxed molds are placed in a burn-out furnace and gradually brought up to a red heat during which cycle any wax residues are burned out and the various phase changes of the refractory occur, assisted by the mineralizing action of the sodium acid phosphate of the binder; and a secondary ceramic bond is formed by the action of the silicic acid and the oxyphosphate cement formed by neutralizing the free phosphoric acid with the refractory cation provided therefor.

In the preceding examples, the refractory powders were proportioned silica sand and silica flour because of the economy and relative low specific gravity of sand. It is to be pointed out that higher quality refractories such as alumina, mullite, zirconia, silicon carbide, and chromium oxide can be utilized as well as the silica products in cases where such cost is permissible. Likewise, I am not limited to zinc salts as the cation agent to take up the free phosphoric acid. I have found that magnesia, aluminum hydroxide, molybdic oxide, or titanium dioxide can be used'in place of the zinc oxide in the refractory powder mix to react with free phosphoric acid. Likewise, magnesium chloride, chromium acetate, aluminum nitrate, copper nitrate, oraluminum chloride, can be used to furnish cations and a volatile acid residue for the binder.

The sodium silicate used as a source of colloidal silicic acid can be .anyof the commercialgra des that range between 1Na,0 to 1.58.0, and 1Na,0 to 4.08.0 and can be diluted with one to four parts of water so that the silica content ranges from about 5 to about 15%. When a 5% silica binder is used, it should be enhanced by l to 2% of an organic binder such as starch, polyvinyl alcohol, or a gum or water soluble cellulose.

Similarly various wetting agents can be used to expedite the wetting of the wax pattern by the investment mixture. The wax patterns can be pre-dipped in a slurry of these binders with silica or zirconia flour to insure a complete coverage of the wax pattern, and the precoat allowed to dry before investing with the investments given in Example I or II above.

It is apparent that when the investment is formed of colloidal hydrated acid binder, refractory powder and salt of amphoteric metal which reacts thermally with the binder during the burning out of the wax so as to render the binder more refractory, such leaves an acidic residue which acts as a residual flux. For example, when neutralized sodium silicate is used it leaves a residue of sodium acid phosphate under such conditions and the sodium acid phosphate residue is a flux for the metal to be cast. When acid chlorides, such as zinc chloride, are used in the binder, the residue is an anhydrous zinc chloride which likewise is a flux for the metal.

WhatIclaim is:

1. A binder for refractory powders comprising colloidal silica formed by neutralizing the alkali of an aqueous sodium silicate solution with sufficient phosphoric acid to peptize the colloidal silica at a pH below 2.0.

2. A binder for refractory powders comprising colloidal silica formed by neutralizing the alkali of an aqueous sodium silicate solution with sufficient phosphoric acid to peptize the colloidal silica at a pH below 2.0 and containing a volatile acid salt of amphoteric high melting point refractory metal at least equivalent to the free phosphoric acid in the binder.

3. A binder for refractory powders comprising colloidal silica formed by neutralizing the alkali of an aqueous sodium silicate solution with at least sufficient phosphoric acid to peptize the colloidal silica at a pH below 2.0 and containing a volatile acid salt of amphoteric high melting point refractory metal selected from the group consisting of the volatile acid salts of aluminum, chromium, copper, magnesium and zinc, and said salt being present in an amount at least equivalent to the excess phosphoric acid in the binder.

4. A colloidal binder for refractories used in the casting of metals, said binder being formed by reacting an aqueous sodium silicate solution with sutficient aqueous phosphoric acid solution to neutralize the alkalinity of the sodium silicate and to peptize the resultant silica sol by sufficient excess acid to hold the pH below 2.0, said acid stabilized silica sol containing, in addition to the free acid, sodium acid, phosphate, and silica sol formed, suflicient polyvalent ion salts of volatile acids in solution to form cementitious polyvalent oxyphosphate with the free phosphoric acid of the binder when the refractory mass is dehydrated and the volatile acid driven off by heat.

References Clted in the file of this patent UNITED STATES PATENTS 1,748,315 'Stoewener Feb. 25, 1930 1,790,272 Leasman Jan. 27, 1931 1,909,008 Prange May 16, 1933 2,072,212 Moosdorf Mar. 2, 1937 2,152,152 Prosen Mar. 28, 1939 2,391,855 Bean Dec. 25, 1945 2,479,504 Moore et al. Aug. 16, 1949 2,680,890 Moore et al. June 15, 1954 2,682,092. Henricks June 29, 1954 

1. A BINDER FOR REFRACTORY POWDERS COMPRISING COLLOIDAL SILICA FORMED BY NEUTRALIZING THE ALKALI OF AN AQUEOUS SODIUM SILICATE SOLUTION WITH SUFFICIENT PHOSPHORIC ACID TO PEPTIZE THE COLLODIAL SILICA AT A PH BELOW 2.0. 